Muscle strength-that is, the fundamental ability of persons without disabilities (7) and with disabilities (1,36) to perform effective and coordinated movements-is considered inefficient in chronic cases of mental retardation (MR) (10). Muscle strength of lower extremities especially is fundamental for the overall health, performance of daily activities, and work-related skills of individuals with MR (20,35).
Individuals with MR present lower levels of muscle strength compared to individuals without MR (13,16,19,38). Low fitness levels of individuals with MR are probably a result of less opportunity for planned and incidental physical fitness activity on a sustained basis. Furthermore, poor physical fitness profiles of individuals with MR have been attributed to factors such as chronotropic insufficiency, poor motivation, and difficulties in accurate fitness assessment of this population (26).
The lower muscle strength of individuals with MR can be associated with a deficiency in the quantity and quality of their muscle tissue, primarily related to their physically inactive life and lack of opportunity to practice skills in school and family settings (3). Inability or unwillingness of some individuals with MR to mobilize their neuromuscular mechanism and produce maximal effort in torque tests is also noted. As a consequence, there is always a possibility for torque indices not to represent their full capacities (35).
Due to the number increase of individuals with MR in society (25) and the reduction of mortality due to science evolution, maintenance and development of muscle strength are important features for the participation of these individuals in productive employment for a long time (9). In addition, the close relationship between physical activity and longevity of individuals with MR (12) causes the need to improve muscle strength through the application of training programs.
Strength increase is associated with 2 basic mechanisms: the metabolic one, which leads to muscle hypertrophy (33), and the neuronal one, which indicates that roughly strength gain is acquired either via agonist muscle activity increase or by antagonist muscle activity decrease. To our knowledge, no information is evident as to what extent strength improvement in people with MR is associated with antagonist muscle activity.
The purpose of this study was to determine the effect of a 4-year systematic basketball exercise program without resistance training on the muscle strength of adults with MR measuring the values of peak torque on a Cybex Norm isokinetic dynamometer (Lumex, Inc., Ronkonkoma, New York, USA) and evaluating antagonist activity. Basketball was chosen because it is a contact sport that includes movements such as sprints, jumps, and quick stops that contribute to muscle strength improvement necessary for injury prevention and performance. It remains an issue, however, for basketball coaches to discover whether basketball training applied to participants with MR without specific instructions applied concerning resistance training can contribute to muscle strength improvement of adults with MR. Studying the effect of basketball training without necessarily focusing on specific fitness components such as muscle strength is expected to advance understanding of significant health issues for individuals with MR.
Experimental Approach to the Problem
This cross-sectional study was designed to address the question of whether a long-term systematic basketball exercise program without resistance training can improve the strength performance of subjects with MR. For this reason 3 groups were selected: untrained adults without MR, adults with MR following basketball training for Special Olympics Games, and sedentary adults with MR. Each group comprised 8 subjects each. To test our hypothesis the lower limb strength of all participants was evaluated because lower limbs are mainly overloaded during a basketball training. The purpose of this study was to investigate the general effect of basketball training on muscle contraction and not on the basketball performance, so we examined concentric, isometric, and eccentric contractions, which are representative of the muscular function.
Three groups of 8 adults each volunteered to participate in this study (n = 24). The first group was composed of 8 adults with normal IQ and constituted the control group (NIQ) (mean age 25.4 ± 0.8 years; mean height 180.1 ± 3.5 cm; mean weight 82.4 ± 4.1kg). All men were college students from the Aristotle University of Thessaloniki participating in sports occasionally.
Individuals with MR were selected according to the following criteria: all participants had mild mental retardation as measured according to the Stanford-Binet Intelligence Scale (42) and they were students of a school of vocational training for adults with disabilities. All individuals with MR were living with their families of the same socioeconomic background (middle class). Furthermore, participants were apparently healthy, with similar height and weight and with no physical or sensory impairments that could interfere with testing protocols. Finally, all individuals with MR were capable of understanding visual and verbal instructions. Thus, all individuals met the same criteria in terms of IQ, socioeconomic background, ability to understand instructions, and absence of health problems prior to the formation of groups, ensuring that the 2 MR groups started from the same reference point prior to basketball training application.
Subjects with MR were assigned to 2 other groups according to their participation in exercise training programs during the past 3 years. Consequently, the second group was composed of 8 trained young adults with MR (MR-T) (IQ range 51-61; mean age 26.5 ± 0.9 years; mean height 177.2 ± 2.3 cm; mean weight 79.9 ± 5.2 kg) who were all basketball players of the Special Olympics Team of Thessaloniki following a basketball training program. Regular basketball training included 3 training sessions per week, 90 minutes for each session, for a period of 4 years continuously. Each training session aiming at a 95 to 145 pulse rate as measured by telemetry (Sport Tester, Finland), consisted of 10 to 15 minutes of warm-up exercises (jogging, stretching, and calisthenics) followed by a 45- to 55-minute period of running with (dribbling, fast brake drills) and without (defensive drills) the ball. Instructional game was also part of each daily session (20-25 minutes). Plyometric, sprinting exercises (10-25 m) and medicine ball throwing were also a regular part of their practice. During the 4-year basketball training period, no systematic strength training program was applied to the MR-T group. Moreover, no individual of this group participated in any form of organized basketball training or any other sport activity during the off-season (July and August). Participants of the third group (MR-R) were 8 young adults with MR who exercised occasionally for recreational reasons (IQ range 48-60; mean age 25.3 ± 1.7 years; mean height 169.0 ± 3.6 cm; mean weight 71.9 ± 4.7 kg), participating only in their physical education school program without taking part in any sport activities of organized form.
Each subject of this sample was informed of the purpose of this study and its experimental risks and procedures. All participants, with and without MR, and parents or legal guardians of MR individuals signed an informed consent document prior to the investigation with all appropriate information related to the testing procedures (risks, difficulties. Etc.) according to the Ethic Committee of Aristotle University of Thessaloniki.
Cybex Norm isokinetic dynamometer was used to measure and estimate the maximal isometric and isokinetic concentric and eccentric torque of the knee extensor and flexor muscle groups. The electromyographic (EMG) activity was recorded using the Neuropack Four Mini device (Nihon Kohden, Tokyo, Japan) connected with a Biopac MP100 Data Acquisition unit (Biopac Systems, Inc., Goleta, California, USA).
The method of the data collection included measurements of the muscle strength for all participants with and without MR. The study included clinical examination, measurement of physical characteristics, and measurement of the IQ of the individuals with MR according to the Stanford-Binet Intelligence Scale (42).
All measurements were conducted at the Department of Physical Education and Sport Science, Aristotle University of Thessaloniki. The clinical examination, measurement of physical characteristics, and measurement of the IQ of the individuals with MR took place at the Laboratory of Developmental Pediatrics and Special Education. Measurements of isometric and isokinetic concentric and eccentric muscle strength were made at the Laboratory of Coaching and Sport Performance. The participants of the NIQ group visited the Laboratory of Coaching and Sport Performance twice to familiarize themselves with the laboratory environment, the equipment, and all the experimental conditions. The participants of the MR groups visited the laboratory 3 times. During the first time, they were familiarized with the environment and the testing procedures, and the other 2 times they were measured so as to confirm reliable results. Instructional procedures were based on demonstration and a total communication approach. Each exercise was demonstrated before its execution to familiarize each subject with its elements. Instructions were repeated until the subject knew what was expected. Each subject was positively reinforced during the entire program to ensure his maximum effort during each exercise session.
Two examiners, both familiar with the operation of equipment and testing procedures, conducted all measurements. The research protocol took place during 2 separate days of testing spaced 48 to 72 hours apart. As the results showed, there was a high reliability (r > 0.90 based on intraclass correlation coefficient [ICC]). Therefore, the design of the study incorporated the estimation of the highest performance of each subject, and the better of the 2 efforts for each parameter was chosen for data analysis.
The subjects performed a typical warm-up consisting of 5 minutes of cycling on a Monark ergometer (Monark Exercise AB, Vansbro, Sweden). They were instructed to follow a self-select cycling resistance and pedal cadence at which they felt comfortable. Then they performed free gymnastic activities that mobilize large muscle groups. For fat-mass estimation, 4 skinfolds (triceps, thigh, abdominal, and subscapular) were measured on the right side of the body using a Lafayette caliper (Lafayette Instruments, Lafayette, IN, USA). The measurement was made with a precision of 0.1 mm. Three sets of measurements were taken. The mean of the 3 measurements was used as the representative value for each skinfold. The equation proposed by Jackson and Pollock (21) was used for the prediction of percent body fat to calculate the fat-free body mass.
An adequate rest period followed and then the main testing session began. The testing procedure was performed from the seated position (hip angle: 115 degrees) and subjects were instructed to fold their arms across their chest. Velcro straps were used to stabilize the trunk, the waist, and the upper thigh on the chair to avoid any movement that could bias the knee torque measurement. The leg was located horizontally to the ground and full extension was checked by aligning a level indicator posteriorly on the medial femoral epicondyle at a point approximate of knee axis of rotation. Gravity correction was estimated according to software program of the instrument. Calibration of the isokinetic instrument was performed according to the procedure recommended by the manufacturer 1 hour prior to each testing session.
For warming up, a series of submaximal isometric, concentric, and eccentric contractions at 60 degrees/sec−1 were performed prior to maximal testing. The participants then performed 3 reciprocal maximal concentric and eccentric contractions and 3 maximal isometric contractions of knee extensors and flexors muscles in a randomized order. The range of motion was from 0 degrees to 90 degrees (0 degrees = full knee extension) with a 3-minute interval between them. Isometric contractions were performed at 25 degrees and 75 degrees of knee extension for knee flexion and extension, respectively. Each isometric contraction lasted 5 seconds. The trial during which the highest peak torque was exerted was used for further analysis.
All subjects underwent the same measurements. The participants were verbally encouraged to perform their best and had permanent visual feedback of their torque scores on the monitor of the dynamometer, as recommended by a previous study (23).
Vastus lateralis (VL) and biceps femoris (BF) EMG were recorded according to SENIAM (surface EMG for a non-invasive assessment of muscles) guidelines (17). Two pairs of bipolar gold electrodes (diameter: 0.8 cm, interelectrode distance: 1.2 cm) were located on VL on the 2/3 midway between the lateral epicondyle and the middle of the ischial tuberosity and on BF halfway between the ischial tuberosity and lateral femoral epicondyle. In addition, the electrode placement was confirmed by palpations of muscle bulk during brief maximal isometric contraction. The proper skin preparation (depilating, abrading, and cleaning with alcohol) reduced the skin impedance to be less than 2 kΩ. The earth electrode was placed on the bony surface on the lateral epicondyle of the other leg. The electrode placement, for minimizing crosstalk, was validated according to the method of Winter et al. (46).
The EMG signals were amplified 1,000× within a bandwidth from 10 to 500 Hz. The amplifier's common rejection mode ratio was higher than 90 db. The signal was then fully rectified and the average EMG (aEMG) values were calculated for a distance corresponding to the period of the constant velocity of the lever arm. The torque, angle, and EMG signals were forwarded with BNS cables to the Biopac, MP100 Data Acquisition Unit (Biopac Systems, Inc., Goleta, California, USA) and the signal was A/D converted using a built-in A/D 12-bit card, with a sample frequency of 1 kHz.
The antagonist activity of VL and BF muscles was expressed as a percentage of the activity of the same muscle when acting as an agonist at the same muscle action, angular velocity, and range of motion (23).
Statistical analysis included the SPSS program for Windows (version 14; SPSS Inc.; Chicago, Illinois, USA). One-way analysis of variance was used to test mean differences between the values of 3 groups. Interaction effects were detected with post hoc analysis. A significance level of 0.05 was used for all tests (p ≤ 0.05).
The NIQ group presented higher absolute torque scores for both knee extensors and flexors than the other 2 groups in all action types tested (p < 0.05). Additionally, the MR-T group presented higher absolute torque scores for both knee extensors and flexors than the MR-R group in all action types tested (p < 0.05) (Figures 1, 2, and 3).
Relative Torque (Torque/L.B.M.)
The NIQ group presented higher relative torque scores for both knee extensors and flexors than the other 2 groups in all action types tested (p < 0.05). Additionally, the MR-T group presented higher relative torque scores for both knee extensors and flexors in all action types tested than the MR-R group (p < 0.05) (Figures 4, 5, and 6).
Both MR groups presented statistically higher antagonistic activity for both knee extensors and flexors than the NIQ group in all action types tested (p < 0.05). The MR-R group presented statistically higher antagonistic activity for both knee extensors and flexors in all action types (p < 0.05) (Figures 7, 8, and 9).
In this study, the results show that the Special Olympics group (MR-T) had significantly higher isometric and isokinetic peak torque values of lower extremities compared to the sedentary MR group (MR-R), indicating that adults with MR present normal training adaptations with respect to peak torque values, as is the case in individuals without MR (32). Furthermore, both MR groups presented lower absolute and relative torque values compared to the NIQ group. Finally, antagonist activity in the Special Olympics group was lower compared to the sedentary MR group and higher compared to the NIQ group.
The findings of this study indicate that the mean isometric and isokinetic peak torque of the lower extremities of young adults with MR, as measured with the Cybex Norm dynamometer, was lower compared to subjects without MR, in agreement with findings of previous studies (14,15,24,25,27). Because the use of a dynamometer is accepted as reliable both for individuals with and without MR (40), inferior torque values of MR groups in this study probably result from many factors such as poor coordination (5), lack of motivation (29,31), lower maximal heart rates that limit cardiac output (14,15), and sedentary lifestyle (15).
Participation of individuals with MR in regular strength training programs including weight equipment, isokinetic or hydraulic resistance, and plyometric exercises without additional weight or resistance has been proved to increase muscular strength in individuals with MR (18,28,30,39,41,43,44). In our study, the MR-T group presented higher torque output compared to the MR-R group. This finding could be explained by the fact that individuals with MR who participated in moderate to vigorous physical activity without resistance training presented better levels of physical fitness (i.e., aerobic capacity, muscle strength) than physically inactive individuals with MR. In other words, participation in community-based physical activity and Special Olympics International programs is associated with better physical fitness in adults with MR compared to other adults with MR who do not participate in Special Olympics International programs or who are not physically active (4,9).
Strength increase is caused either by hypertrophy or by neuronal factors (33). Hypertrophy measurement was not in the scope of this study, which specifically focused on the neuronal mechanism and especially in the antagonist activity.
The synergy of agonist and antagonist muscle regulates the resultant torque in each joint angle. The existing literature explained thoroughly that antagonist activity contributes to the joint stability, especially during intense muscle contraction (37). Additionally, antagonist activity depends on many factors such as muscle action and training effect. Concerning the effect of action mode, previous studies have shown that antagonist activity during eccentric contraction is lower than isometric and concentric ones (2,6) as a result of the different strategy of muscle activation that occurs in eccentric contraction compared to others (11). Furthermore, previous studies have shown that increased muscle strength levels after training cause a decrease in the antagonist activity (33). In this study, the fact that healthy people had lower antagonist activity compared to the active MR group and the active MR group had lower antagonist activity than the sedentary MR group could be explained by their exerted torque difference. Besides, learning effect can explain the coactivation difference between active and inactive MR subjects (22) considering that basketball practice includes isometric eccentric and concentric actions.
In conclusion, the findings of this study indicate that systematic participation in basketball without the application of a resistance training program can improve muscle strength levels of individuals with MR.
Because of low levels of muscle strength, persons with MR are unable to adequately perform activities of daily living and have limited job opportunities (15). Physical activity in Special Olympics International program with proper intensity, frequency, duration, and length of training seems to provide an effective mode of training and performing continuous and sustained work for people with MR. Because individuals with MR are most typically required to use more physical than cognitive skills in the workplace, enhanced physical capabilities can make a significant contribution to the overall vocational and social development of these individuals (45). Thus, a greater number of individuals with MR should participate in training programs offered by Special Olympics International.
We are grateful to the group of young adults with and without MR for their participation in this study. Furthermore, we want to express our gratitude to the families, coaches, and physical fitness teachers of adults with MR for their cooperation and unfailing support during basketball research. We would also like to thank the entire laboratory and medical staff for their hard work for the completion of this study.
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